While attending Bugil High School in Cheonan, Chungcheongnam-do, Ko was a starting pitcher in the team.
During his last year in 2008, Ko developed into an ace pitcher for the team, posting 3-3 with a 2.44 ERA in 55.1 innings pitched.
His first KBO league win came as a starter on May 12 against the Kia Tigers, tossing six innings and surrendering only one run on eight hits.
Ko compiled a 2–2 record and a 2.11 ERA as of May 30, when he hurled eight shutout innings against the LG Twins and only surrendered three hits in that span.
After the 2010 season, Ko was selected for the South Korea national baseball team to compete in the Intercontinental Cup held in Taichung.